This present invention relates to compression, hermetic glass-to-metal seals and current feed-throughs. Such seals are commonly used as feed-throughs for signal conductors in hermetically sealed, metal cased capacitors, relays and other electronic components.
The invention has particular application in the manufacture of hermetic glass-to-metal seals for capacitors which handle high amperage current pulses, which may be AC or pulsating DC. It has been realized that hermetic glass-to-metal seals introduce heating and power loss when used as feed-throughs for signal conductors handling high amperage pulsating currents.
The purpose of this invention is to set forth improved compression-type, hermetic glass-to-metal seals which will provide significantly lower heating and power loss when used in high amperage pulsating current applications than prior art.
The following discussion will describe the manufacture of a compression, hermetic glass-to-metal seal, used in a metal cased hermetically sealed capacitor. However, it will be understood that the formation and use of a compression, hermetic glass-to-metal seal in accordance with the present invention is widely applicable to analogous structures, metal cased relays and other hermetically sealed electronic components.
The art of manufacturing compression, hermetic glass-to-metal seals is well-known and has been described by Mayer in U.S. Pat. No. 3,035,372. Such seals comprise an outer metal compression ring which contains an annular glass button through which is disposed a coaxial conductor assembly including an outer tube or eyelet and an inner conductor. Commonly the inner conductor is plain copper wire of a suitable gauge to carry the currents of the circuits involved. The outer tube or eyelet which surrounds the wire is used to obtain the essential glass to metal bond which is needed for hermeticity. Such compression seals are commonly manufactured in different manufacturing operations. In a first, high temperature stage, the ring, glass button, and tube are fired to form a unitary seal assembly. This assembly is mounted to a can such as a cylindrical tube containing the electrical components such as a capacitor such that the lead wire from the latter protrudes through the eyelet of the preassembled seal. The capacitor case and seal are then soldered together in the second stage at a temperature much lower than the initial manufacture of the seal. The conductor is soldered to the eyelet in a similar manner.
Prior art calls for the outer member (1) to be made of common steel the glass member (2) to be selected from the potash soda or potash lead type and the inner member (3) to be made of nickel 52, an iron nickel alloy containing about 52% nickel and 48% iron.
Examination of the thermal expansion properties of the materials shows that the outer ring (1) has higher thermal expansion than either the glass or central member so that the glass is placed in radial compression. Nickel 52 has a coefficient of thermal expansion slightly lower than the glass. The result is that the outer ring (1) transmits radial compression into the glass (2) which is transmitted to the inner member (3). For glasses of the type mentioned such as Corning 9010 or 9013 a good bond can be made to nickel 52. The physical strength and hermeticity of the just described prior art is excellent, and the physical integrity and electrical performance for conduction of steady state direct electrical current of the just described seals is excellent.
With the increasing use of pulsating and alternating current in circuit design it has become evident that a significant loss of power occurs when high frequency signals must pass through the 52 alloy eyelet via the copper conductor. When sufficient signal energy exists, the eyelet temperature can possibly increase up to the point of melting the soft solder joint which is used to hermetically seal the copper lead wire conductor to the eyelet.
The resultant drain of power from these electronic circuits requires that the circuits deliver more power. This results in increased weight and size for these electronic or electrical devices and equipment to overcome the losses due to the the hermetic seal effects.
The losses are primarily ferro-magnetic and partially due to skin-effect or eddy current.
To overcome these losses we investigated available non-magnetic materials which could match suitable sealing glasses and have found that Hastelloy B alloy (consisting essentially of 65% nickel and 30% molybdenum) possesses an acceptably low coefficient of thermal expansion.
The construction of a seal with Hastelloy B proved entirely satisfactory and is the preferred embodiment of the present invention. Hastelloy B is a trademark of Cabot Corporation of Kokomo, Ind.
Investigation has shown that compression, hermetic glass-to-metal seals introduce heating and power loss when used as feed-throughs for signal conductors handling high amperage pulsating currents. These effects are now found to result from induction heating of the nickel 52 inner member or eyelet.
The cause of the heating is not precisely known, and might be falsely attributed to the construction of the capacitor or to dielectric heating of the glass seal or to any of the other components. An isolation test was performed in which it was discovered that the small nickel 52 eyelet having a conductor passed therethrough carrying a current was readily heated, particularly if the inside diameter of the aperture were relatively close to that of the diameter of the wire, as in seals of the present type. This heating mechanism was felt to be directly analogous to RF induction heating (i.e., magnetic induction heating, and eddy current induction heating, or some combination). Substitution of a non-magnetic stainless steel eyelet in this experiment resulted in virtually no heating whatsoever. However, a compression seal of this type cannot be constructed to use these results since stainless steel is unsuitable in glass to metal hermetic bonds because of its coefficient of thermal expansion is too high. A non-magnetic metal having a good capability of bonding to glass was desired; and also having a property of an acceptable thermal expansion coefficient. Of all alloy materials readily available, only the alloy Hastelloy B (consisting essentially of 65% nickel and 30% molybdenum) possessed an acceptably low coefficient of thermal expansion, was known to be non-magnetic and makes a good bond to glass. The construction of a seal with Hastelloy B proved entirely satisfactory and is the preferred embodiment of the present invention.
The selection of a material for use as the inner eyelet member is critical and such a material must have the following properties:
1. The material must be non-ferromagnetic. PA0 2. The material must possess good chemical bonding properties during the sealing operation. PA0 3. The coefficient of thermal expansion must be compatible with the glass to maintain hermeticity. PA0 4. The elasticity characteristics of the alloy should be compatible with the glass so as to allow equalization of compressive forces created without setting up unduly high stesses. PA0 5. The material must be relatively inexpensive.
It has been found that the alloy commonly known as Hastelloy B* has the properties required and is entirely suitable and the preferred metal alloy for use in the present invention. Hastelloy B is an alloy of about 64% nickel and 28% molybdenum with other percentages of alloying material being limited to about 1/2% carbon, 5% iron, 2.5% cobalt and 1% chromium. This alloy is widely used for its outstanding corrosion resistance. The coefficient of thermal expansion, of Hastelloy B is about 100 cm/cm.times.10.sup.-7 /.degree.C. and will bond very well to glasses such as Corning 9013 and 9010 which have thermal coefficients of expansion of about 95 cm/cm.times.10.sup.-7 /.degree.C.
Testing has also shown that the use of a non-ferromagnetic outer ring further reduces the effects of induction heating.
In the present invention a non-magnetic outer ring made of monel or non-magnetic stainless steel 304 is used to reduce induction heating. The inner member is a hollow or solid tube. The glass is Corning 9013 and 9010.
In the specifications, the following abbreviations may be used in the following descriptions.
Nickel=Ni PA1 Molybdenum=Mo PA1 Tungsten=W PA1 Hastelloy=Hastelloy B, TM of Cabot Corporation. (Other Hastelloy trademarked products are not included.) PA1 Platinum=Pt PA1 Palladium=Pd
The following units are used through this description: coefficient of linear expansion-cm/cm/.degree.C..times.10.sup.-7. Percentage compositions of alloys given herein in weight percent (w/o), in accordance with common engineering practice. 300 Series steels, such as 304, refer to AISI designations.
Before any investigation to solve this problem was made, consideration was given to the use of copper-to-glass seals. Copper-to-glass seals, however, are not practical for the following reasons: the type of glass used with copper seals is soluable in solutions used in plating. Copper must be plated prior to the sealing step to ensure a good metal-to-glass bond. In addition, copper-to-glass seals do not maintain hermeticity after being exposed to temperature cycling.